Weak constraint 4DVar for 1D Burgers equation

[JHopeCollins]

Requires firedrakeproject/firedrake#3870

1. Linear scalar advection examples using "observations" generated from analytical solutions with perturbed initial conditions.
   Shows setting up strong and weak constraint systems using both pyadjoint and firedrake, assuming you already have the observations to hand.

• Forward model example.
• Strong constraint using a pyadjoint.ReducedFunctional.
• Strong constraint using a firedrake.FourDVarReducedFunctional.
• Weak constraint using a pyadjoint.ReducedFunctional.
• Weak constraint using a firedrake.FourDVarReducedFunctional.

2. Burgers equation examples using observations generated from a known "ground truth" initial condition.
   Shows setting up the 4DVar systems using both pyadjoint and firedrake, including generating your own ground truth observations from known data.

• Strong constraint using a pyadjoint.ReducedFunctional.
• Weak constraint using a pyadjoint.ReducedFunctional.
• Weak constraint demo using firedrake.FourDVarReducedFunctional - only builds and minimises the system, meant to be a fairly static demo showing current use.
• Weak constraint script using firedrake.FourDVarReducedFunctional - also does some other things like optionally running taylor tests, doing a single RF evaluation. Mostly used for development and debugging.